1. Technical Field
The present invention relates to a droplet discharging method and a droplet discharging apparatus.
2. Related Art
As an example of a droplet discharging apparatus, an ink jet printer which records (prints) an image by discharging ink droplets on various recording media such as a sheet or film is known. The ink jet printer alternately repeats, for example, a dot forming operation of, while moving (scanning) a head provided with a plurality of nozzles in a main scanning direction, discharging ink from each of the nozzles onto a recording medium and a transporting operation of moving (transporting) the recording medium in a sub-scanning direction perpendicular to the main scanning direction to form dots (dot rows) which are lined up in the main scanning direction to be lined up in the sub-scanning direction, thereby forming an image on the recording medium.
In this type of ink jet printer, as a method for further increasing a recording speed, a method of increasing the number of nozzles has been employed. Specifically, this is a method of increasing the number of dots discharged in a single scanning process by increasing the number of nozzles per head or lining up a plurality of heads to increase a recording speed. As a high-speed recording method, for example, there is a method (band recording method) of forming an image by completing image formation of a region (band) having a width corresponding to the length of a row constituted by a plurality of nozzles through a single scanning process, transporting a recording medium in a sub-scanning direction by the width, forming bands to be lined up in the sub-scanning direction so as to allow the end portion of the formed band to abut the end portion of the band formed by the subsequent scanning process, and repeating the above operations. In the case of the band recording method, there may be a case where striped patterns (banding) are formed along the boundary between the bands. This is caused by variations in accuracy of transport in the sub-scanning direction, an effect of the difference between the characteristics of switching parts of nozzle rows, and the like.
In JP-A-6-47925, as a method of suppressing the degradation in image quality due to the banding, a method of performing partial overlap is suggested. POL control is a method of dispersing variations in characteristics or accuracy, and specifically, as the simplest example, is a method of causing the lower end region of a band formed by a scanning process of a head and the upper end region of a band formed by the subsequent scanning process of the head to overlap each other and forming dots of a part of the lower end region of the initially formed band and dots of a part of the upper end region of the subsequently formed band to overlap each other in the same region. In JP-A-10-323978, a method of forming a high-quality image by using a plurality of nozzle rows is suggested.
However, in the case where the plurality of heads provided with a larger number of nozzles are configured to be lined up in the sub-scanning direction in order to achieve higher speed, there is a problem in that there is a difference (for example, difference in concentration or the like) between images of the regions of the upper and lower end portions of the recording medium in the sub-scanning direction (recording start portion and end portion) and an image of the region of the center portion thereof, and as the difference is visually recognized, image quality deteriorates. Specifically, since the heads are configured to be lined up in the long region in the sub-scanning direction, there may be a case where partial overlap between the plurality of heads cannot be performed on the upper and lower end portions of the recording medium although partial overlap between scanning processes is possible. That is, there is a problem in that POL control between heads having different characteristics may not be performed on the upper and lower end portions. As a result, there may be a case where there is a difference between images of image regions formed only by the heads corresponding to the upper and lower end portions and an image of an image region subjected to the POL control between the plurality of heads. In addition, there is a problem in that, in a transition region between the regions of the upper and lower end portions and the region of the center portion, the difference between the heads in characteristics affects the image as the number of heads being used is increased (the difference between the upper and lower end portion regions and the center portion region cannot be smoothly transitioned).